Photographic elements made predominantly of silver chloride, with minor amounts of silver bromide and iodide (e.g., &gt;70% chloride), are known in the prior art. These elements have wide processing latitude and can be made and utilized for most of the art fields which employ silver halide as the sensitive medium. However, since silver chloride-containing elements are much slower than those containing mainly silver bromide, the use of such elements has been generally limited to graphic arts applications, e.g., contact, low-speed camera films, etc. Since silver chloride is generally more soluble than the other silver halides, processing of exposed elements is more conveniently done. It would be desirable to use this benefit in many of the other silver halide art fields.
It is well recognized in the art that silver chloride strongly favors the formation of cubic crystals having {100} crystal faces. In the majority of photographic emulsions silver chloride crystals when present are in the form of cubic grains. With some difficulty it has been possible to modify the crystal habit of silver chloride. Claes et al, "Crystal Habit Modification of AgCl by Impurities Determining the Solvation", The Journal of Photographic Science, Vol. 21, pp. 39-50, 1973, teaches the formation of silver chloride crystals with {110} (rhombododecahedral) and {111} (octahedral) faces through the use of various grain growth modifiers. Wyrsch, "Sulfur Sensitization of Monosized Silver Chloride Emulsions with {111}, {110}, and {100} Crystal Habit", Paper III-13, International Congress of Photographic Science, pp. 122-124, 1978, discloses a triple-jet precipitation process in which silver chloride is precipitated in the presence of ammonia and small amounts of divalent cadmium ions. In the presence of these cadmium ions, control of pAg (the negative logrithim of silver ion concentration), and pH resulted in the formation of rhombododecahedral, octahedral, and cubic crystal habits, presenting grain faces lying in {110}, {111}, and {100} crystallographic planes, respectively.
It is thus apparent that the photographic art has been limited to a few methods for the preparation of useful octahedral chloride-containing emulsions. Octahedral, silver chloride-containing grains with {111} crystal surfaces are of practical importance because they present a unique surface arrangement of silver and halide ions, which in turn influences the grain surface reactions and adsorptions typically encountered in photographic applications (e.g., gold, sulfur, and dye sensitization). In addition, theory teaches that the surface structure also influences interstitial silver ion concentration and the ionic space charged layer which can have pronounced effects on photographic speed.
There is a need to prepare a suitable octahedral grain emulsion wherein the grains of the emulsion are at least 50 mole percent chloride and are photographically useful without the use of a cadmium compound and in the presence of a new and effective organic grain growth modifier compound.